Since these no longer follow directly from scroller.clientHeight/Width.

Use these to ensure messages for current line are scrolled into view
in demo/widget.html.

Closes #802

Instead of a single 'inline' option, add noHScroll and coverGutter options.

This seems to be a better fix for the scroll artifacts.

Closes #795

There's apparently a timing issue with adding widgets to
a just-initialized editor.

The off pun is rather awful, but it does have precedent, and is
more succinct.